Interferometric near-field apparatus based on multi-pole sensing

ABSTRACT

An apparatus suitable for providing near-field measurements of a workpiece. The apparatus comprises a source of electromagnetic radiation for generating an incident wave; means for directing at least a portion of the incident wave to the workpiece; a probe tip acting as an antenna and capable of re-radiating a signal wave, said signal wave developing as an interactive coupling between said workpiece and said probe tip; means for creating an interference signal based on the signal wave and a reference wave; and a detector for interrogating at least one of the phase and amplitude of the interference signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly assigned U.S. patent application Ser. No.08/511,166 entitled "An Interferometric Detecting/Imaging Method BasedOn Multi-Pole Sensing", and to commonly assigned U.S. patent applicationSer. No. 08/511,579 entitled "An Interferometric Measuring Method Basedon Multi-Pole Sensing", which applications arc filed on even dateherewith and incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to a method for deriving physical properties of aworkpiece, the method realizable by way of a novel interferometric nearfield microscope.

INTRODUCTION TO THE INVENTION

We are investigating method and instrumentation suitable for deriving ordetermining physical properties or a workpiece.

By the phrase "a workpiece", we mean, preferably, materials that may bevariously characterized as organic compounds, conductors,semiconductors, insulators, ferromagnetics, paramagnetics, diamagneticsor composites thereof.

By the phrase "determining physical properties of a workpiece", wereference the following concepts.

It is known that there can be many different kinds of interactionbetween electromagnetic radiation and a workpiece. For example, somecommon interactions include refraction, reflection or absorption. For aspecific workpiece, these phenomena are usually described quantitativelyin terms of phenomenological parameters empirically assigned to theworkpiece: e.g., index of refraction, and absorption index.

Further, by considering the properties of electromagnetic waves, onecan, for example, correlate the index of refraction with the possibilityof polarizing a material (i.e., separating positive and negativecharges) as expressed by a dielectric constant of the workpiece;magnetize a workpiece (i.e., line up magnetic dipoles) as expressed bythe permeability of the workpiece; or, correlate a particular kind ofoptical absorption with the electrical conductivity of the workpiece.

In sum, accordingly, by the phrase "determining physical properties of aworkpiece", we reference methods and instrumentation suitable fordetermining phenomenological properties of a workpiece including, interalia, polarization, magnetic or dielectric susceptibility, or dielectricconstant of the workpiece.

SUMMARY OF THE INVENTION

Classical analytical techniques suitable for determining physicalproperties a workpiece, even at the molecular level, are known, andinclude e.g., scanning microscopies, acoustic imaging, x-ray analysis orelectron imaging. Implicit in these known classical techniques, however,are several subsumed measurement assumptions or requirements: forexample, that an average measurement is based on a workpiece thatcomprises an initial mass of at least several molecules; or, ameasurement, particularly in obtaining optical information, is delimitedin the sense that the resolution capabilities of classical opticaltechniques i.e., confocal, fluorescent and polarized light microscopies,are diffraction limited and the sensitivity can not reach single atomdetection.

We note that most of these analytical techniques, in particular, NearField Scanning Optical Microscopy, NSOM, are based on fluorescencedetection, whose signal detection may be limited by a moleculardetection efficiency corresponding to the fraction of molecules in thesample that can actually be detected.

There are several important limitations on the magnitude of a signalthat can ultimately be obtained from a single-molecule. A finitefluorescence lifetime and photobleaching, combined with typical photondetection efficiency, yield typically a maximum of a few thousanddetectable fluorescent photons. Molecular detection probabilities (andconcentration detection limits) are strongly influenced by thebackground (for example, Raman and Raleigh scattering from contaminantsor solvents, laser intensity inhomogeneity . . . ) and signal photocountamplitude probability distributions, as well as the average S/N. Theresponse to one or several molecules in a given probe volume is clearlyhighly dependent on photocount statistics, just as the moleculardetection efficiency. Complications such as excitation laser noise,molecular diffusion and other processes (e.g., incompletephotobleaching) can also obscure a distinction between one or twomolecules.

In current experimental setups, the instrument components are chosen tominimize the background signals (for example, small illumination volume)and to maximize the light collection efficiency (for example, highnumerical aperture objectives). Typically, the best current NSOMconfigurations allow a spatial location of λ/10, limiting efficientdiscrimination among different molecules.

The last decade has seen relevant applications in modern imagingtechniques mainly due to improvements in optics, instrumentation andcontrast enhancement techniques. There has been a rapid growth inmicroscale imaging, spectroscopic measurements to the nanometric scale,and microfabrication. Scanning probe microscopy is increasingly beingapplied in materials science, chemical and biological surfaceapplications, due to its nondestructive nature, and capabilities for usein fluids. Typical examples of scanning probe microscopes includeScanning Tunneling Microscopes (STM), Atomic Force Microscopes (AFM) andall derivatives, for example, the Magnetic Force Microscope (MFM) or theThermal Probe Microscope.

More recently, advances in the optics of nanometer dimensions haveoccurred, particularly with increasing emphasis on scanning near-fieldoptical microscopy (NSOM). Typically, the influence of a local probenear an interface can result in variations of the distribution ofelectromagnetic fields. There are various configurations (reflection andtransmission modes) of these near-field sensing techniques, exploitingradiation effects based on far-field detection of optical interactionsbetween probe and workpiece.

Among various NSOM optical sensors, spherical particles, and pointedtips may be used to perturb an incident radiation, e.g., an evanescentwave. The probe sensor can create a field distribution which may bedetected in the far-field, carrying information on the near-field zonesurrounding the probe source and characteristic or its opticalproperties. In many of the experimental NSOM setups, an optical scanningsensor incorporates a small aperture (see description or aperture-basedsystem as NSOM probes in U.S. Pat. Nos. 4,917,462; 5,272,330) from whicha local excitation electric field distribution can emerge fortransversally illuminating the workpiece. As a result, near-fielddistortions clue to the interaction between the aperture acting as aweak light source ( e.g., low efficiency of about 50 nW for 80 nmaperture) and the workpiece are similarly detected in the far-field bymeasuring the intensity only of the radiation (i.e., the square or theelectric field in place of a direct measurement of the electric field asdescribed in the present invention) with the use of a photon counterdevice.

Our work is cognizant of the significance and importance of methods andinstrumentation suitable for determining physical properties of aworkpiece. In particular, we build on the classical analyticaltechniques and challenge extant assumptions or requirements, to therebyqualitatively extend and redefine them.

The present invention can provide a significant extension in spatialresolution to at least λ/500 and even down to the atomic level, therebyproviding an ability for lower concentration detection limits, forexample, below the ferromolar level. In addition to being highlysensitive, the present interferometric imaging/sampling method, based onsensing a multi-pole coupling between a probe and a workpiece throughamplitude and/or phase detection, permits one to process complexsusceptibility measurements or the complete excitation spectrum or asingle-molecule. In particular, a coupling interaction comprises acomplex function including a resistive or imaginary component and areactive or real component (inductive or capacitive) that can bemeasured down to the atomic level. Moreover, a unique ability ofexcitation spectroscopy combined with background minimization, throughinterferometric coherent detection can overcome several intrinsiclimitations or fluorescent based techniques for single-moleculardetection.

The present invention describes a method in which an electromagneticradiation interaction between a probe dipole and a workpiece dipoleexternally driven by an incident field, e.g., an evanescent orpropagating or standing field, can be detected by interferometricallymeasuring amplitude and phase differences between a reflected wave and areference wave component. The dipole-dipole coupling mechanism comprisesa main source of contrast generation, and the near field interferometricdetection of the scattered optical power can achieve ultimate S/N andresolution due to a (ka)³ signal dependence, rather than (ka)⁶ inconventional NSOMs.

The following theoretical part of this disclosure demonstrates howphysical and chemical information, preferably optical-spectroscopicinformation, about source probe and workpiece, can be obtained down tothe atomic level. As the near-field zone surrounding the probe ischaracteristic for its dielectric/magnetic properties, and as the probevolume can have atomic dimensions, it becomes clear that this method canprovide specific information and/or images never before realizable.

We now disclose a method suitable for deriving physical properties of aworkpiece, the method comprising the steps of:

1) sampling an electromagnetic wave packet representative of workpieceproperties and comprising encoded wave information derivable from amulti-pole interactive coupling between a probe tip and the workpiece;

2) decoding said electromagnetic wave packet by interrogating at leastone of its phase and amplitude information;

and

3) correlating this information to referent physical chemical propertiesof the workpiece.

The method as defined can realize important advantages. Its enablementallows one to measure sundry workpiece properties, for example, itscomplex susceptibility with a very high spatial resolution down toatomic dimensions.

A preferred realization or this method is now disclosed below, andfeatures utilization of a novel interferometric near field apparatusproviding superresolution e.g., 1 nanometer resolution, thereby enablingresolution of a workpiece close to the atomic level. The novel apparatuscomprises:

1) a source of electromagnetic radiation for generating an incidentwave;

2) means for directing at least a portion of the incident wave to theworkpiece;

3) a probe tip acting as an antenna and capable of re-radiating a signalwave, said signal wave developing as an interactive coupling betweensaid workpiece and said probe tip;

4) means for creating an interference signal based on the signal waveand a reference wave:

and

5) a detector for interrogating at least one of the phase and amplitudeof the interference signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated in the accompanying drawing, (not drawn toscale), in which:

FIGS. 1, 2 show in overview a principle of interferometric measurementusing an apparatus comprising multi-pole interactive coupling;

FIG. 3, 4 provides schematics for explaining basic concepts about apreferred interferometric near-field apertureless microscope operatingin a transmission and in a reflection mode, respectively:

FIG. 5 illustrates measured optical dipole coupling vs. probe workpiecespacing, compared with theory:

and

FIG. 6 shows an alternative embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

We develop the detailed description by first disclosing aninterferometric near-field apparatus that is preferably employed inrealization of a measuring method based on a multi-pole sensing. To thisend, attention is directed to FIG. 1, which shows in overview such ageneralized apparatus operating in a transmission mode.

The FIG. 1 apparatus 10 comprises a source 12 of electromagneticradiation, preferably generating an incident electric field E_(i),preferably in the optical spectrum, for example from UV to IR. Theelectric field E_(i) is directed through a conventional interferometer14 to a focusing clement 16 which preferably comprises an aperture or anobjective lens. The interferometer 14 may comprise e.g., a Michelson,Fabry-Perot or Twyman-Green apparatus. The driving electric field E_(i)is now focused on a workpiece 18, in turn supported by a transparentsubstrate 20. (Note that in an alternative embodiment shown in FIG. 2comprising a reflection mode, the driving electric field E_(i) isfocused directly on the workpiece 18).

FIG. 1 also shows a probe tip sensor 22 preferably placed with respectto the workpiece 18 such that a distance between the probe tip 22 and atleast a portion of the workpiece 18 surface is smaller than the source12 radiation wavelength, or a multiple of it.

Note that a suitable probe may comprise a sharp metallic tip or anuncoated silicon and/or silicon nitride tip, or a tip coated with aconductive layer or a molecular system. The probe preferably comprises ahigh refractive index material. A near-field probe capability may berealized by e.g., a scanning tunneling microscope (STM), an atomic forcemicroscope (AFM), an aperture or apertureless near-field opticalmicroscope, a near-field acoustic microscope, a thermal microscope or amagnetic force microscope (MFM). A notion of "scanning" references thefact that probe and workpiece may be in relative motion. Reference maybe made for example to U.S. Pat. Nos. 5,319,977; 4,343,993; 5,003,815;4,941,753; 4,947,034; 4,747,698 and Appl. Phys. Lett. 65(13), 26 Sep.1994. The disclosures of each of these patents and publications areincorporated herein by reference.

The FIG. 1 probe tip 22 is capable of re-radiating, in the form of asignal beam -SIG- (E_(s) +E'_(r)), an incident radiation beam, thesignal beam comprising carrier beam E'_(r) combined with workpiece 18property information encoded in the scattered field E_(s) as thetip-feature dipole-dipole coupling. The signal beam -SIG- comprises ascattered electromagnetic field variation wave E_(s), due preferably tothe probe 22 vibrating (or moving relatively) in close proximity to theworkpiece 18 surface. Note that the FIG. 1 signal beam SIG illustrateswhat is summarized above as an electromagnetic wave packetrepresentative of workpiece properties and comprising encoded waveinformation derivable from a multi-pole interactive coupling betweenprobe and workpiece. In particular, the incident radiation E_(i) candrive this action such that a dipole 24 dipole 26 coupling interactionis created between tip dipole 24 and workpiece dipole 26.

FIG. 1 further shows that the aperture 16 helps for creating aninterference signal -IS- based on the signal beam (E_(s) +E'_(r)) and areference beam (E_(r)), and for directing the interference signalthrough the inteferometer 14. The output signal 28 of the interferometer14 can measure either the amplitude of (E_(s) +E'_(r)) or its phasedifference with a reference beam E_(r). Note that a self-interferencephenomena can be alternatively exploited and comprises spatiallyseparating the beam 12 in several components having phase differencesthat are subsequently made to interfere.

As alluded to above, FIG. 2 shows the FIG. 1 apparatus, but modified forutilization in a reflection mode. One difference in the change of modeis that in the reflection mode, the substrate 20 does not need to betransparent: the apparatus of FIG. 2, otherwise, may be realized mutarismutandis visa vis the apparatus of FIG. 1.

As illustrated in the fundamental FIG. 1, it should be noted that theincident light can be directed to a near-field probe either through theworkpiece (transmission mode) or by reflection at its surface. In thislatter case, particular attention has to be taken to discriminate theprobe signal against spurious reflected light. For the sake ofsimplicity, only a transmission case is described below.

Attention is now directed to FIG. 3, which shows details of a preferredapparatus 30 for the realization of the present method and which areconsistent with the generalized FIGS. 1, 2 apparatus 10.

The FIG. 3 apparatus 30 comprises an interferometer and includes thefollowing components: an electromagnetic source, preferably a tunablewavelength (e.g., 400 nm<λ<2500 nm) laser 32, an optional acousto-opticmodulator 34 in order to prevent spurious back reflection of light:generating laser noise; a special beam expander 36 for relative beam andmeasurement area movement; an aperture 38; a means for splitting anincoming lightwave into first and second lightwaves comprising apellicule beam splitter 40; a polarising beam splitter 42; atransmission/collection optics (preferably a Nomarski Oil/dark-fieldobjective) 44; a Wollaston prism 46; and, 3 photodetectors PD_(n). FIG.3 shows in association with the interferometer an optical probe sensorand a set of electronics 48 (enclosed by the broken-border box in FIG.3) that permits both topographic measurements and probe-workpiecedistance Feedback regulation with at least nanometer accuracy.Preferably using an AFM Feedback, one can therefore image a surfacetopography while simultaneously recording a near-field optical image.

In the FIG. 3 illumination pathway, the laser beam of appropriatepolarisation first passes through the beam steering unit 36 in order toexpand the beam size in accordance with the objective aperture 44. Thelaser beam can be adjusted continuously within the beam steering unit 36by preferably using a piezoelectric positioning system (e.g., x-y-z PZTtube) allowing small motion with nanometer accuracy. The beam steeringis also controlled by an image-collection electronics 48 for relativelypositioning the focused spots, a measurement area of the workpiece 50and an optical probe sensor 52 while the scanned beam is traced back andforth.

The expanded laser light passes through the aperture 38 (preferablymatching geometries of the transmission/collection optics 44) in whichan angular discrimination of the incident radiation distributionpreferably selects a total internal reflection illumination. Typically,the pellicle beam splitter 40 reflects about 10% of the incidentradiation to a reference arm of the interferometer 30 to a detector,preferably a photodiode PD_(R) and transmits about 90% of the incidentradiation to the polarising beamsplitter 42.

The beam preferably is imaged to a plane wave that overfills the backaperture of the Nomarski objective 44 which focuses the plane wave totwo diffraction limited spots in the workpiece 50. Because the aperture38 blanks the illumination near the center of the beam, the excitinglight wave propagates as an evanescent wave in the area illuminated inthe workpiece.

When a probe sensor 52 that can operate various motions relative to theworkpiece 50 at different frequencies (e.g., resonance frequency) withthe help of a three-coordinate pizoelectric translator 54, is approachedtypically a few nanometers close to the workpiece 50, the probe 52 iscapable of locally perturbing the wave impinging the smallest spatialasperity (e.g., the very end of a pointed STM or AFM tip) of the probe52 resulting in a coupling mechanism between the probe dipole andfeature dipole of the workpiece.

In terms of an electromagnetic field distribution, the scatteredelectric field variation due to the vibrating and scanning probe tip 52in close proximity to the workpiece 50, may be measured by encoding itas a modulation in the phase of a second arm of the interferometer 30.

As it is shown in FIG. 5, the optical signal strongly depends on thedistance of the probe 52 dipole from the workpiece 50 dipole. Theoptical signal is collected by the objective 44 and reflected throughthe polarising beamsplitter 42 to a Wollaston prism 46 with its axisoriented relative to the Nomarski prism, in order to optimize theinterference of the reflected electric fields from the two spots, and tomeasure the phase of the signal beam (E_(s) +E'_(r)) which correspondsto the real part of the scattered wave E_(s).

The light continues through an external lens 56 that focuses the lightonto a photodiode PD_(A) and PD_(B). The imaginary part of the scatteredwave E_(s) can be detected by orienting the Wollaston prism 46 axis tobe aligned with the Nomarski prism 44 axis, so as to separately detectthe optical powers in the two spots (without mixing) in the differentialphotodiode PD_(A-B). This detection arrangement preferably operates atpre-selected frequencies ranging from 100 Hz to 100 MHz.

The output signal of this differential detector preferably is sent to anoise suppressor 58 for further noise improvement, by combining thephotocurrent from PD_(A-B) with that from the reference photodetectorPD_(R) which is fed a sample of the incident beam. The noise suppressoroutput preferably is sent to a lock-in amplifier input 60 in order todemodulate the resultant near-field AC signal carrying interestinginformation about workpiece properties. The output of 60 can be sent toa controller/computer -CC- device for generating an optical image thatcan be preferably compared simultaneously with an attractive mode AFMimage generated by the independent feedback loop system 48.

The scattered field E_(s) from the probe tip 52 end will in general bepresent on top of a spurious background of light scattered from the tipshank. The background signal can preferably be reduced in these ways.First, we use a confocal arrangement for optical illumination anddetection; this restricts the detection region to within 100 nm of thetip end. Second, if the tip is modulated in z at frequency f_(z) by anamplitude which is approximately the tip radius, the back-scatteredlight from the tip end will have a larger modulation on the workpiece ascompared with light scattered from regions that are farther away as thetip is approached very close to the workpiece. Finally, one can furtherenhance the signals at the spatial frequencies of interest (i.e.,corresponding to the radius of the probe tip) by vibrating the workpieceinternally by approximately the tip radius at frequency f_(x) anddetecting the interferometer signal at the sum frequency (f_(x) +f_(z))as it is illustrated by the broken line in the FIG. 3 box 48.

In an alternative embodiment, we can replace the FIG. 3 elements 34, 36,38 and 44 preferably with a tapered metallized single-mode optical fiberelement such as described by R. E. Betzig et al. in U.S. Pat. No.5,272,330 and incorporated by reference herein.

In a particular reflection configuration, the experimental arrangementshown in FIG. 4, apparatus 62, can incorporate, for example, directlythrough the objective 44, an optical feedback system for monitoring thetip-workpiece surface distance. The probe tip 52 can, for example, beresonantly vibrated with the aid of the piezo actuator 54 and thevibration amplitude can be detected with the help of a second laser beam64 at a given wavelength different from that of laser source 32. Theoptical feedback preferably comprises an assembly of optical elements,for example, lenses 66 and 68 and dichroic filters 70, 72 in order todiscriminate the two optical paths of different wavelengths fordirecting selected light onto the appropriate detectors PD_(n). Byadjusting the optics 66, 68 and 70 with respect to the probe tip 52, onecan ensure that the light impinging the rear face of the probe 52 doesnot interfere with respect to the light impinging the very end of theprobe tip 52. The optical detection of the back reflected light from therear face of the probe 52 is then directed through the electronic set ofbox 48. The stable operation of the feedback system requires a properchoice of imaging parameters (e.g., scan rate . . . ) and lightdistribution (e.g., focus and alignment of said second laser beamrelative to said first beam) in order to minimize any laser noiseaffecting output signal quality.

As articulated above, we have developed the detailed description of thenovel method of the present invention by first disclosing preferredinterferometric near field apparatus (FIGS. 1-6). Utilization of suchapparatus can yield information about workpiece properties. We now turnour attention to how this information can be abstracted in anintelligible manner, to thereby demonstrate the utility of theinterferometric near-field apparatus.

Since the optical dipole interaction varies as r⁻³, a measured signalprimarily derives from the tip end. One can therefore assume that thetip can be modeled as a sphere of radius a, and polarizability α_(t) andthat the feature that is being imaged on the workpiece has apolarizability α_(f) and radius a (although the theory could easily begeneralized for any arbitrary feature size). If the tip and workpieceare immersed in a driving electric field E_(i) (caused by the incidentradiation), and ε is the dielectric permittivity of the surroundingmedium, the following coupled equations for the induced dipole momentsP_(t) and P_(f) in the tip and feature respectively (FIG. 1) can bewritten as:

    P.sub.t =α.sub.t ε(E.sub.i +E.sub.f)         (1)

    P.sub.f =α.sub.f ε(E.sub.i +E.sub.t)         (2)

Here, E_(t) and E_(f) are the corresponding near-fields generated by thedipole moments of tip and feature respectively. For the case where thespacing r between tip and feature is greater than the diameter 2a, thedipole approximation can be used and the following expressions for E_(t)and E_(f) can be written: ##EQU1## Substituting for E_(t) and E_(f) inequations (1) and (2) and solving for P_(t) and P_(f) it is found, aftereliminating terms of order less than r⁻³, that: ##EQU2##

Equations (5) and (6) show very clearly how the tip polarization coupleswith the feature polarization to generate a polarization modulation term##EQU3## It is this polarization modulation that produces a modulationin the scattered electric field E_(s) as the tip--feature spacing r ismodulated by vibrating the tip. As mentioned earlier, equations (5) and(6) are derived for the case where r>2α the linear dimension of thedipole (for a sphere, this linear dimension is comparable to itsdiameter). The corresponding equations for arbitrary r can be obtainedusing quasi-static theory simply by changing r to ##EQU4## in equations(5) and (6). More general expressions for the polarization modulation ΔPand the polarizability modulation Δα are thus: ##EQU5##

From equations (7) and (8), ΔP and Δα decreases rapidly from theirmaximum values as the tip-feature spacing is increased ##EQU6## As itwill be described later, for situations where ##EQU7## being the opticalpropagation constant in a medium of refractive index n) the scatteredelectric field modulation ΔE_(s) is directly proportional to Δα; one cantherefore expect to see a strong decrease in ΔE_(s) as the tip-featuredipole-dipole coupling decreases with increasing r. As shown in FIG. 4,experiments show a rapid variation over tip-workpiece spacingscomparable to the tip radius. Furthermore, equations (7) and (8) showthat ΔP and Δα are proportional to the product of the complexpolarizability of the tip α_(t) and that of the feature α_(f).Consequently, the phase of the scattered field component ΔE_(s) canchange drastically depending on the complex polarizability of thetip-end as previously observed (FIG. 5).

Now, the modulation ΔE_(s) of the scattered field E_(s) caused by thepolarizability modulation Δα can be calculated by applying thescattering matrix treatment used by van de Hulst (Light Scattering bySmall Particles, Wiley, New-York 1957) to study light scattering fromsmall particles. For an incident field E_(i), the spherically scatteredwave has electric field E_(s) at a distance d in the far field given by##EQU8## where the relevant scattering matrix component S (which hasboth real and imaginary components) can be written in terms of thepolarizability α: ##EQU9## and for a simple sphere of radius a, andcomplex refractive index m (relative to the surrounding medium)##EQU10##

Note that imaginary terms of order k⁵ and higher order terms in theexpansion for S have been omitted as we are dealing with scattering fromvery small particles (i.e., ka<<1).

As just shown, the reflected wave from the back surface of the workpiece50 is a concentric spherical wave of amplitude ##EQU11## where ##EQU12##is the optical spot radius and NA is the numerical aperture of theobjective lens. The expected phase difference Δφ between reference andsignal beams is then E_(s) /E_(r) or Δφ=5k³ αN A² /8π. The reflectedwave is phase advanced by π/2 with respect to the scattered wave. Thusfrom equations (9) and (10) the imaginary component of S will give riseto a scattered field E_(s)φ that is π/2 phase delayed with respect toE'_(r) generating an overall phase shift, and the real component of Swill give rise to a small scattered field E_(se) that is π out of phasewith respect, to E'_(r) generating an overall extinction.

Let us first consider the case where α is purely real (i.e., m is real(silicon) or m is imaginary (gold)). The z-vibrating probe tip producesa modulation ΔS, ΔE_(s)φ and ΔE_(se) respectively. ΔE_(se) interferesdestructively with E'_(r) to produce a fractional extinction ##EQU13##of the reflected power in the spherical wave E'_(r). Using equation (9)and the expression for E'_(r) it becomes: ##EQU14##

From the second term in equation (10) Re [S] (and Re [ΔS]) vary as (ka)⁶and ##EQU15## will yield a negligibly small signal as the probe sizedecreases substantially below 50 nm. This term is in fact the fractionalpower scattered by the particle--i.e., what is typically detected inNSOM's.

By contrast, in an interferometric system ΔE_(s)φ gives rise to a phaseshift ##EQU16## in the reflected beam. This phase change Δφ produces afractional power change ##EQU17## of 2Δφ at the photodiode (due to thedifferential phase detection system). ##EQU18##

Therefore from equation (10), this fractional power change varies onlyas (ka)³. It is this dependence that gives reasonable S/N ratios atsub-molecular resolution in the present method and eventually, thepossibility to achieve atomic resolution.

One can estimate the ultimate resolution that may be achieved with theFIG. 3 apparatus using some simple considerations. Taking a silicon ormetal tip (i.e. m²¹) of radius a and polarisability α, we have,##EQU19## For a coherent, shot noise limited phase detection system with1 mW laser power, we can show that Δφ_(min) ≃10⁻⁸ rad/√Hz. This wouldsuggest that for He-Ne laser light (λ=633 nm) with NA=0.85, a≃1.7angstroms, i.e., the resolution should reach the atomic level.

Now consider the general case where m is complex, α is complex.Returning to equation (10) and neglecting the second term, (as we areonly concerned with resolving features substantially below 50 nm), Swill have both a real and imaginary part that produce scattered fieldsE_(s) that vary as (ka)³. Note that whereas in a dark-field measurement(like typical NSOM's), one will again be measuring scattered fractionalpowers that vary as (ka)⁶, in SIAM, the imaginary part of α will producea fractional power change that varies as (ka)³ as it is evident fromequation (12); these power changes can be detected by orienting theWollaston prism axis to be aligned with the Nomarski prism axis, so asto separately detect the optical powers in the two spots (withoutmixing) in the differential photodiode. Other work on light scatteringfrom plasmon resonances in spheres and more recently from STM tips arebased on dark-field detection of the scattered optical power--i.e. (ka)⁶--signal dependence--and as before run into severe S/N problems atresolutions below 50 nm. However, the use of an interferometric system,as reported herein, demonstrates the ability to achieve the ultimate S/Nand resolution.

Combining equations (12) and (13) with equations (8) and (9) and notingthat the polarizability is related to the susceptibility χ by ##EQU20##equations (12) and (13) can be written in terms of the susceptibilitiesχ_(t) and χ_(f) of tip and feature respectively: ##EQU21##

Therefore, both the real and imaginary parts of the susceptibility of afeature can be determined--in principle down to the atomic scale--withtwo simultaneous measurements; the tip susceptibility being measuredindependently using a known reference surface as the workpiece.

Note, in conclusion, that the disclosure corresponding to equations(1-15) can be developed, routaris mutandis, for a case herein anexternal driving field comprises a magnetic field and induces magneticdipole-dipole coupling, and for a case wherein an external drivingelectromagnetic field comprises both appreciable electrical and magneticfield components for inducing electromagnetic dipole-dipole coupling.

We claim:
 1. An apparatus suitable for providing near-field measurementsof a workpiece, said apparatus comprising:1) a source of electromagneticradiation for generating an incident wave; 2) means for directing atleast a portion of the incident wave to the workpiece; 3) a probe tipacting as an antenna and capable of re-radiating a signal wave, saidsignal wave developing as an interactive coupling between said workpieceand said probe tip; 4) means for creating an interference signal basedon the signal wave and a reference wave; 5) a detector for interrogatingat least one of the phase and amplitude of the interference signal;andmeans for positioning at least one of the probe tip and the workpiece inclose proximity such that the distance between the probe tip and theworkpiece can create a multi-pole coupling interaction.
 2. An apparatusaccording to claim 1, wherein the incident wave is generated by atunable source from UV to IR for driving a multi-pole coupling betweenthe workpiece and the probe tip.
 3. An apparatus according to claim 1,wherein the incident wave is generated at a fixed wavelength.
 4. Anapparatus according to claim 1, wherein the incident wave comprises anevanescent wave.
 5. An apparatus according to claim 1, wherein theincident wave comprises a standing wave.
 6. An apparatus according toclaim 1, wherein the incident wave comprises a propagating wave.
 7. Anapparatus according to claim 1, wherein the incident wave is generatedby a coherent electromagnetic source.
 8. An apparatus according to claim1, comprising a focusing element for directing at least one portion ofthe incident wave to the workpiece.
 9. An apparatus according to claim8, wherein the focusing element comprises a high numerical aperture lensfor a total internal reflection illumination of the workpiece.
 10. Anapparatus according to claim 1, wherein the probe tip comprises a highrefractive index material.
 11. An apparatus according to claim 1,wherein the probe tip comprises a spherical particle.
 12. An apparatusaccording to claim 1, wherein the probe tip comprises a pointed tip. 13.An apparatus according to claim 1, comprising a Nomarski objective forcreating the interference signal.
 14. An apparatus according to claim 1,wherein at least a portion of the incident wave functions as thereference wave.
 15. An apparatus according to claim 1, comprising aMichelson type interferometer for creating the interference signal. 16.An apparatus according to claim 1, wherein the detector comprises adifferential photodiode assembly.
 17. An apparatus according to claim16, wherein the detector can operate at pro-selected frequencies rangingfrom 100 Hz to 100 MHz.
 18. An apparatus according to claim 1, whereinthe means for positioning comprises at least one piezoelectricpositioner.
 19. An apparatus according to claim 18, comprising ascanning probe microscope electronic feedback system for controlling thepiezoelectric positioner.
 20. An apparatus according to claim 1, whereinthe distance between the probe tip and the workpiece is smaller than theincident wavelength.
 21. An apparatus according to claim 1, comprisingan optical fiber for directing the incident wave to the workpiece. 22.An apparatus according to claim 1, comprising means for operating theapparatus in a transmission mode by using a workpiece transparent to theincident wave, or using a transparent substrate to support theworkpiece.
 23. An apparatus according to claim 8, comprising means foroperating the apparatus in a reflection mode by positioning the probetip on the same side as the focusing element with respect to theworkpiece.
 24. An apparatus according to claim 1, comprising means forgenerating a time variable multi-pole interactive coupling by modulatingat least one of the wavelength of the electromagnetic wave packet, therelative positioning of probe and workpiece, and using an externalapplied electromagnetic field to the interaction region.